1. Field of the Invention
The present invention relates to a crosslinkable acrylic rubber composition exhibiting a superior balance between the storage stability and cure rate, and having only minimal risk of corroding metals or being contaminated by metals.
2. Description of the Background Art
Acrylic rubber is an elastomer (co)polymer containing alkyl acrylate and/or alkoxyalkyl acrylate as principal components. The acrylic rubber has excellent oil resistance, heat resistance, weather resistance, ozone-proof properties, and permanent compression set, and the like. Rubber compositions containing this rubber have conventionally been used as sealing materials such as packing and gasket, and various molded articles.
Such an acrylic rubber possesses cure sites such as, for example, halogen-type cure sites, epoxy-type cure sites, and diene-type cure sites. An appropriate crosslinking agent or crosslinking accelerator corresponding to the type of cure site is added when the rubber composition is prepared.
Particularly, as crosslinking agents for a halogen-containing acrylic rubber, a composition comprising a fatty acid metal soap and sulfur or a sulfur donor, a composition comprising a triazine derivative and a dithiocarbamine acid derivative, and the like are known. Of these, the crosslinking agent comprising a fatty acid metal soap is inferior in the rate of crosslinking, permanent compression set, and water resistance; the crosslinking agent containing triazine derivative also exhibits only an insufficient rate of crosslinking, anti-scorching properties, and storage stability. Moreover, metal corrosion and metal contamination during crosslinking are problems with the crosslinking agent containing triazine derivative.
To solve these problems, Japanese Patent Publication No. 39181/1980, for example, proposed a composition comprising 1 an alkali metal salt of an organic carboxylic acid, 2 sulfur or a sulfur donor, and 3 an urea compound, a thiourea compound, an imidazoline compound, or an amino acid compound. This crosslinking agent, however, exhibits only a retarded rate of crosslinking and involves permanent compression set.
There is also some prior art disclosing triazine derivative-containing crosslinking agents. For example, Japanese Patent Application Laid-open No. 180539/1983 discloses a crosslinking agent which comprises 1 trithiocyanuric acid and 2 an alkali metal salt organic carboxylic acid. Japanese Patent Application Laid-open No. 320456/1993 discloses a composition containing 1 triazine thiol compound, 2 a quaternary ammonium salt, and 3 an alkali metal salt of carboxylic acid, and a composition containing a a reaction product of a triazine thiol compound and a quaternary ammonium salt. These crosslinking agents, however, are also inadequate in respect of storage stability, anti-scorching properties, rate of crosslinking, and the like. A method for improving anti-scorching properties of a halogen-containing rubber composition which comprises a triazine thrithiol crosslinking agent by adding a reinforcer and/or a filler with a pH of about 2-7 has been disclosed by Japanese Patent Publication No. 82413/1993. This composition cannot exhibit a satisfactory rate of crosslinking.
In addition, with regard to prevention of metal corrosion during cross-linking, a method of adding a hydrotalcite to a halogen-containing acrylic rubber (for example, Japanese Patent Application Laid-open No. 150409/1990) and a method of adding a silica-based reinforcing filler and a hydrotalcite to a halogen containing acrylic rubber (for example, Japanese Patent Application Laid-open No. 228746/1995) have been proposed. These rubber compositions, however, are also inadequate in respect of the balance between the storage stability and the rate of crosslinking.
Any conventional halogen-containing acrylic rubber compositions thus have not reached a satisfactory level in terms of well balanced characteristics such as storage stability, rate of crosslinking, permanent compression set, metal corrosion, metal contamination, and the like.
The present invention has been achieved in view of these problems in the conventional technology and has an object of providing a crosslinkable halogen-containing acrylic rubber composition exhibiting superior balance between the storage stability and rate of crosslinking, free from the risks of metal corrosion and metal contamination, and exhibiting superior tensile strength, permanent compression set, water resistance, and the like.